The present invention relates to a reading lens system suitable for use with facsimiles, image scanners and like apparatus.
Lens systems used with facsimiles, image scanners and like apparatus must have a resolving power of approximately 7 .mu.m on the image side and hence are required to provide an extremely good contrast at high frequencies. Further, in order to minimize the loss of light in an inner peripheral portion of an image circle formed by the reading lens system, a nearly 100% aperture efficiency is needed. In addition, it is necessary to achieve satisfactory compensation for distortion. To meet these requirements, Gauss type lenses are conventionally used, as described, e.g., in Unexamined Published Japanese Patent Application Nos. 50-62631, 59-90810, 60-181716 and 63-75720.
A problem with the conventional Gauss type lenses, which are used with an F number on the order of 1:4.0 and a half viewing angle of about 20.degree., is that their performance in a sagittal direction is not given as much consideration as in a meridional direction. If one wants to improve the overall image quality by a suitable method such as increasing the reading density in a sagittal direction, good lens performance is required in both meridional and sagittal directions. However, this requirement has not been completely met by prior art lens systems.